1. Field of the Invention
This invention relates to an article for improving the effectiveness of rotary drum or wheel trenching machines employing carbide-tipped cutting bits for pulverizing or trenching concrete, asphalt roadways and similar surfaces. More particularly, the invention relates to articles for protecting the support blocks, in which rotatable carbide cutting bits are mounted, from premature failure caused by impact and abrasion from pieces of pulverized material encountered during operation of the machines using the bits.
2. Description of Background Art
When concrete or asphalt roadways, aircraft runways, and the like require a trench opening, replacement or complete removal, a machine frequently employed for this purpose is a rotary drum pulverizer or wheel cutter. Typically, such machines utilize a large drum or wheel which has a diameter of two to ten feet, and a cutting width ranging from three inches to twelve feet. Welded to the outer circumferential surface of the drum or wheel are 60 to 300 or more steel support blocks which hold cylindrical, tungsten carbide-tipped cutting bits. The cutting bits extend radially outward from the circumferential surface of the drum. A typical rotary pulverizer drum or wheel is rotated by a 400 horsepower motor at a linear speed of 1,500 surface feet per minute.
The cutting bits used in many rotary pulverizers are elongated, solid cylinders, typically 31/2" long by 3/4" in diameter at the shank end. The outer or top face of the cylinder typically has a conical-shaped tungsten carbide tip brazed to it. The shank of each cutting-bit is held within a longitudinally disposed bore in a separate forged steel support block welded to the circumference of the pulverizer drum or wheel. Approximately one-half of the length of the bit extends outward from the top, upper transverse face of the holding block. Each cutting-bit is secured in its support block with a clip or sleeve that permits the cutting-bit to rotate freely about its cylindrical axis. This permits the bit to be rotated by tangential frictional contact with the material which it is cutting. As a result of rotation of the bit, wear of the bit is more evenly distributed, extending the useful life of the bit. U.S. Pat. No. 4,201,421 discloses a split sleeve for rotatably mounting cutting bits in their support blocks.
Although providing the capability for free rotation of the cutting-bit results in more uniform wear and extended life of the bit, wear of the bit support block continues to be a problem. The flat, upper transverse face of the bit support blocks is continuously impacted with abrasive materials during the operation of the rotary drum pulverizers or wheel machines. Also, if a bit wears down to the extent that it extends only a short distance out from the face of its support block, more rapid and destructive wear of the support block occurs. Excessively worn bit-support blocks must be removed from the pulverizer drum or wheel with a cutting torch, and a new support block welded onto the drum or wheel. This replacement process is time consuming, and therefore, costly. Furthermore, it frequently happens that replacement of support blocks under field conditions results in a misalignment of the bore axis of the holding support block from its optimum orientation, decreasing the effectiveness of machine operation.
To alleviate the problem of cutting-bit holding block wear, I introduced in October of 1983 an accessory which I referred to as the "Spin-Shield." This accessory is comprised essentially of a flat, hardened steel annular ring or washer which is adapted to fit between the enlarged base of a cylindrical cutting bit, and the transverse outer face of the support holding block which rotatably supports the bit. In addition to absorbing wear which would otherwise be experienced by the bit holding block, my "Spin-Shield" provided other advantages. One such advantage is the separation provided between the cutting-bit chamfer and the bit support holding block, preventing the formation of a bur on the inside of the bore of the holding block. Another advantage is the reduction of cutting-bit friction, allowing the cutting-bit to rotate more freely. This results in cooler operation and more even wear of the cutting-bit, substantially extending its life.
Subsequent to my introduction of the "Spin-Shield" bit holding support block protector, I observed in the field a cutting-bit with a bell-shaped integral flange near the middle of the forged bit shank. The apparent purpose of the bell-shaped flange was to achieve in a limited way some of the advantages of my "Spin-Shield."
After developing my "Spin-Shield," I devised a "Pulverizer Cutting Bit Shield" to protect holding blocks for cylindrical cutting bits from excessive wear. That cutting-bit comprised an annular steel ring shaped like an inverted dish with a central hole and a substantially curved, convex upper surface The ring is adapted to being attached coaxially over the upper portion of the cutting-bit into the bottom entrance of the central hole. The upper surface of the ring is then pounded down on the shank until it abuts an upper annular flange onto the shank of the bit, and is retained in position by a tight interference fit between the outer diameter of the shank and the inner diameter of the ring.
The pulverizer cutting-bit shield described above is an effective means for protecting cutting-bit holding blocks from excessive wear. Moreover, the rotation of the pulverizer cutting-bit in unison with the shield fixed to its shank is particularly effective in applications where it is desired to mix material such as soil impacted by the cutting-bit.
However, I have found that for some applications of pulverizer cutting bits, it would be desirable to have a freely rotatable cutting-bit shield. In particular, for those applications of pulverizer cutting bits in which high impact forces and/or highly abrasive materials are encountered, a freely rotating bit shield would be better because normal wear is distributed evenly on the cutting-bit, thereby extending the useful life of the bit.
Accordingly, I developed a novel and highly effective rotatable cutting-bit shield, which resulted in the issuance to me of U.S. Pat. No. 4,660,890, Apr. 28, 1987, "Rotatable Cutting Bit Shield." Field testing of the aforementioned rotatable cutting-bit shield has proven it to be highly effective in protecting the faces of holding support blocks in which cutting bits fitted with the rotating shield are installed from premature failure. For some applications, however, it would be desirable to provide additional means for protecting cutting-bit support holding blocks, in which a variety of conventional cutting bits may be mounted. With that goal in mind, I conceived of alternate protective structures which could be fastened to the outer transverse face of cutting-bit holding support blocks.
The purpose of these structures would be to protect the hardened steel holding support block from abrasion and impact damage of concrete pieces, rocks, pieces of pavement, etc. brought into abrading impact with the holding support blocks during the pulverization or cutting process.
To accomplish its intended function, a pulverizer cutting-bit holding support block face protector must possess two characteristics: (1) It must be very hard, to resist abrasion wear and damage; and (2) It must withstand high impact forces without shattering.
In the past, hardened steel rings have been attached to the outer transverse faces of cutting-bit holding support blocks, to decrease wear rates of the holding support blocks. Hardened steel shield rings are relatively easy to attach to steel cutting-bit holding block faces, by welding. Moreover, hardened steel rings of the appropriate hardness (Rockwell 40 or higher) have very good resistance to shattering by impacting objects. However, many of the materials being cut which are encountered in the normal operation of a pulverizer have a higher surface, or Moh's scale, hardness than hardened steel. Such hard materials will quickly wear down the surface of a hardened steel shield ring, limiting the effectiveness of hardened steel as a shield material. Steel protection rings with greater hardness, above 50 Rockwell, tend to shatter when encountering normal high impact forces.
With these limitations in mind, I experimented with shield rings fabricated from tungsten carbide, reasoning that the carbide, being harder than all materials normally encountered in the operation of a pulverizer, would afford acceptable abrasion resistance.
As predicted, the carbide shield rings which I fabricated and attached to the faces of cutting-bit holding blocks provided excellent abrasion resistance properties. The carbide holding block shield rings which I tested required more development to solve the problem of holding support block wear, for the following reasons.
First, the attachment of a fixed carbide shield ring to a steel holding support block requires high localized heating of both the ring and block to a very high temperature, typically over 1,500.degree. F. This high temperature causes the whole holding block to become annealed, resulting in a substantial reduction in surface hardness, and a corresponding diminution in abrasion resistance of the portion of the holding support block not covered by the carbide shield ring. Also, the strength of the support holding block is lowered as a result of being heated to a high temperature in a localized area.
A second problem encountered in attempting to use solid carbide rings as holding block shields results from the large difference in the coefficients of thermal expansion for carbide and steel, respectively. Thus, steel, having a coefficient of thermal expansion approximately eight times greater than that of carbide, builds up large stresses in an attached carbide ring, when the two materials are heated and cooled while being attached to one another. These large stresses ultimately can result in cracks causing fracturing and early catastrophic failure of shield rings fabricated from solid carbide.
In addition to the drawbacks described above of using holding support block shield rings fabricated from solid carbide, the carbide itself has low impact resistance. Thus, solid carbide shield rings are subject to breaking upon being impacted by pulverized pieces of rock, concrete or the like, of sufficient size and kinetic energy.
Having encountered the above-described problems in providing cutting-bit holding support block protection by means of solid carbide shields, I undertook to devise a more satisfactory solution to the problem of protecting cutting-bit holding support blocks.
One such cutting-bit holding block shield which I devised and tested consisted of an annular cup-shaped hardened steel container filled with a composite material consisting of tungsten carbide chips held in a german-silver matrix. This shield showed early promise in the laboratory testing. However, field testing under the much more severe conditions encountered in the actual use of pulverizing machines revealed shortcomings of the composite shield. The malleability of the softer matrix material caused it to peen under rock impact. This caused the matrix material within the annular steel container to flow radially inwards and outwards under repeated impacts, buckling the container walls and causing early failure of the shield. Even case hardening the steel container did not solve the problem. Hardened steel washers were used in various combinations with the composite-filled container, but did not significantly increase the wear life of the shield. Accordingly, a new approach to the problem was conceived of.